The invention relates to circuit board modules having multiple integrated circuit packages. More specifically, the invention relates to the inductive properties of circuit board modules coupled with leads of differing lengths.
Memory modules within computer or other electronic systems often consist of a printed circuit board (PCB) with integrated circuit (IC) packages containing the memory circuits attached to the PCB. These memory modules can take various forms known in the art, for example, single in-line memory modules (SIMMs), dual in-line memory modules (DIMMs), and other forms. The modules are coupled to a motherboard having system components such as one or more processors, buses, I/O devices, etc., by an interface that has multiple leads to make an electrical contact with the memory module.
The leads of the memory module interface have associated electrical properties including inductance, which is proportional to a resistance to current flow provided by the leads. Inductance causes a current lag in which current flow lags the corresponding voltage change. For example, when a voltage level changes on a memory module interface lead, the inductance of the lead affects the timing with which the corresponding current changes. Because current is required to change the states of the transistors within the memory module ICs, inductance can affect the operation of the memory module.
As clock frequencies within electronic systems increase, the voltage changes used to communicate the clock frequencies become closer together in time and the impedance of the leads can become a factor in operational frequency. Specifically, a sufficiently high operating frequencies, the inductance of a module and associated leads can become a limiting factor in the speed at which the module can operate.